摘要:

The role of the boron nucleus in nucleosynthesis is discussed. Because of the fragile nature of boron nuclei, they are destroyed rather than created in cosmological settings such as the big band or stellar burning cycles. This instability also severely constrains the buildup of heavy elements in both processes. However,8B is an important intermediary in stellar burning, as its beta‐decoy neutrinos are primary objects of searches for solar neutrinos in experiments on earth. Interactions of galactic cosmic rays with the interstellar medium are believed to form Nature’s10B and11B, as well as6Li and9Be. The success of this model leads to speculations concerning the expanding universe.

ISSN:0094-243X    

DOI:10.1063/1.40869

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We discuss the energy bands, lattice constants, atomic positions, charge density contours, cohesive energies, formation enthalpies, and free energies of B12C3, B13C2, and B12obtained fromab initioself‐consistent pseudopotential calculations.

ISSN:0094-243X    

DOI:10.1063/1.40861

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Complete structural analyses of some boron and many boron‐rich phases were hampered for many years by the difficulties posed by disorder and partial occupation of sites. Thus, for example, the most recent reinvestigation of the &bgr;‐rhombohedral boron unit cell concluded that 320 atoms occupy 20 different symmetry‐inequivalent atomic positions, of which six are partially occupied, while in the case of &agr;AlB12the unit cell is reported to contain 13.0Alatoms statistically distributed over five interstitial sites around a boron framework containing 8B12icosahedra, 4B19units and 4 singleBatoms. In this paper we use electronic structure calculations to reveal the driving force leading to occupation of interstitial positions: the bare boron framework would be electron deficient and the role of the extra atoms is formally to donate sufficient electrons to fill the covalent bonding states of the boron framework. Results for the electronic structure and bonding in a range of other phases are presented, from the simpler icosahedra‐based structures such as &agr;‐rhombohedral boron, boron carbide, andLiAlB14, through toAlB31and the properties of transition‐metal dopants in &bgr;‐rhombohedral boron.

ISSN:0094-243X    

DOI:10.1063/1.40874

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The Jahn‐Teller effect is shown to be the fundamental reason of the general distortion of the B12icosahedra in molecular and crystalline structures. In consequence of the Jahn‐Teller effect the degeneracy of the upermost partly occupied energy level is lifted, and occupied and unoccupied states become separated. This explains the semiconductor behavior of the boron‐rich solids with icosahedral structure elements, and moreover it allows to interpret their structure‐overlapping electronic properties consistently.

ISSN:0094-243X    

DOI:10.1063/1.40840

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A self‐consistent‐field, near‐Hartree‐Fock method is applied to study the structure and stability of B4C. Hydrogens serve as bond tie‐offs. Consideration of a large variety of B12C2H16cluster configurations reveals that one C is highly favored over 0, 2, or 3 in the icosahedron. The C‐B‐C chain is highly favored over all other chains. C’s as far apart as possible are favored, as are C s in triangle positions. The combination of a B11C icosahedron (C in a polar triangle position) with a C‐B‐C chain is the lowest in energy. Geometric optimization is used for the two most‐stable conformations. Two‐icosahedron clusters are also investigated.

ISSN:0094-243X    

DOI:10.1063/1.40855

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The optimized C2vgeometry of ortho‐carborane, 1,2‐C2B10H12, is determined from Hartree‐Fock calculations. For this geometry, a carbon atom is substituted for a boron atom at one of the 4 inequivalent boron sites and the ground‐state unrestricted Hartree‐Fock eigenvalues and molecular orbitals are found. One thus obtains the valence structure of the B(1s) core‐excited molecule according to the Z+1 approximation. The eigenvalue of the highest occupied molecular orbital is then subtracted from the experimental B(1s) ionization energy of the same site in ortho‐carborane. This determines the excitation energy of the most tightly bound exciton for that site. Three of the sites yield nearly identical excitation energies of 191.9 eV; the fourth site yields an excitation energy of 190.9 eV.

ISSN:0094-243X    

DOI:10.1063/1.40812

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The de Haas‐van Alphen (dHvA) effect of HfB2has been studied using a field modulation technique. Single crystal samples for the dHvA measurements were grown by a floating zone technique of rf heating. Observed dHvA oscillations in the (101¯0), (112¯0), and (0001) planes have frequencies of the order of 102T. The values and angular dependence of the dHvA frequencies quite closely resemble those of an electron Fermi surface (FS) of ZrB2. It is proposed from an analogy with ZrB2that HfB2has a ring‐like electron FS around the K point in the Brillouin zone on which nearly ellipsoidal FSs are joined together and is a semi metal having carriers of 0.04 electrons (and holes) per unit cell though the oscillations corresponding to the hole FS could not be observed due to the sample limitation.

ISSN:0094-243X    

DOI:10.1063/1.40820

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Self‐consistent calculations of the electronic band structure of zirconium diboride, ZrB2, have been performed. The total and individual band charge densities are analyzed in terms of their site (zirconium or boron) symmetry and plotted. The total charge density shows very little charge asymmetry about either site.

ISSN:0094-243X    

DOI:10.1063/1.40785

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

General concepts of hopping‐type transport and localization are reviewed. Disorder, electronic correlations and atomic displacements, effects ignored in electronic band structure calculations, foster localization of electronic charge carriers. Examples are given that illustrate the efficacy of these effects in producing localization. This introduction is followed by a brief discussion of the relation between hopping‐type transport and localization. The fundamentals of the formation, localization, and hopping transport of small polarons and/or bipolarons is then described. Electronic transport in boron carbides is presented as an example of the adiabatic hopping of small bipolarons. Finally, the notion of vibrational hopping is introduced. The high‐temperature thermal diffusion in boron carbides is presented as a potential application of this idea.

ISSN:0094-243X    

DOI:10.1063/1.40803

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The temperature and frequency dependences of the dielectric functions and ac conductivities (&sgr;ac) of several boron carbide compositions in the single phase region (9–20 at %C) have been investigated. The low temperature (<25 K) responses are found to be characteristic of phonon‐assisted hopping transport. Evaluation of the various contributions to the real part of the dielectric constant shows that the electronic contribution is relatively large (&Vegr;’∞&bartil;6) whereas the lattice contribution is small (&Vegr;’1≊0.2–2.0), a situation akin to that of the highly covalent semiconductors. This result implies that the electron‐lattice interaction in these materials is of short range. The dipolar contribution, &Vegr;’d, which arises from the hopping motion of localized carriers, dominates the response at low frequencies. &sgr;acexhibits a power law dependence on frequency, &sgr;(&ohgr;)=A&ohgr;s, where s=0.85–0.90 at 4 K and decreases with increasing temperature. The results are discussed in terms of various hopping models.

ISSN:0094-243X    

DOI:10.1063/1.40805

出版商:AIP    

年代:1991

数据来源: AIP